Multi section laser diodes are well known in the art and can be switched between different wavelengths. Typically the diode is calibrated at manufacture to determine the correct control currents that should be applied in each section of the laser so as to effect the desired output frequencies from the laser.
One of the first known multi-section laser diodes is a three-section tuneable distributed Bragg reflector (DBR) laser. Other types of multi-section diode lasers are the sampled grating DBR (SG-DBR), the superstructure sampled DBR (SSG-DBR) and the grating assisted coupler with rear sampled or superstructure grating reflector (GCSR). A review of such lasers is given in Jens Buus, Markus Christian Amann, “Tuneable Laser Diodes” Artect House, 1998 and “Widely Tuneable Semiconductor Lasers” ECOC'00. Beck Mason.
FIG. 1 is a schematic drawing of a DBR 10. The laser comprises of a Bragg reflector section 2 with a gain or active section 6 and phase section 4. An anti-reflection coating 9 is sometimes provided on the front and/or rear facets of the chip to avoid facet modes. The Bragg reflector takes the form of Bragg gratings 5. The pitch of the gratings of the Bragg reflector varies slightly to provide a Bragg mode which moves in frequency through varying the current supplied to these sections. The optical path length of the cavity can also be tuned with the phase section, for example by refractive index changes induced by varying the carrier density in this section. A more detailed description of the DBR and other tuneable multi-section diode lasers can be found elsewhere, for example in the publication by Jens Buus, Markus Christian Amann, entitled “Tuneable Laser Diodes” Artect House, 1998.
As detailed above, such tunable semiconductor lasers contain sections where current is injected to control the output wavelength/frequency, mode purity and power characteristics of the device. Various applications in telecommunications and sensor fields require that the laser can operate at points in a predetermined frequency/wavelength grid. Moreover, many applications require the power output of the device to be within a defined tolerance for each operating point, and in general, the operating points must be distanced from mode jumps and have high side-mode suppression. In order to provide lasers for such applications, each individual device must be characterised to the desired specification, so there is a corresponding need for a system or algorithm to map the output of the laser over a range of operating currents. It will be appreciated that for characterisation of lasers in production environments, such a system should also be fast, reliable and automated.
Certain types of tunable lasers present a characteristic where in certain conditions the output wavelength and power of the laser is dependent on the previous state of the laser. This characteristic is known as hysteresis. If an operating point of the laser is selected in a region where the laser has this characteristic, the output of the laser will be indeterminate and cannot be guaranteed, as it will be dependent on the previous operating point of the laser. It is known to provide applications that identify regions of instability in a laser, so as to ensure that the laser does not operate in these regions. One such application is disclosed in International Patent publication No. WO00/49692 entitled “A method of Evaluating Tuneable Lasers”. This publication discloses a method wherein a reflector section of a laser is injected with varying current, while different constant currents are injected into the remaining tuneable sections of the laser. The method requires that for each set of different constant currents injected into the remaining sections of the laser, the current in the reflector section should be first swept in one direction, and then in the opposite direction back to the start value of the reflector current, while at the same time the power output of the laser device is measured and stored. The difference between the power measured for the same value of reflector current in the two opposite directions is then calculated and stored. Every combination of different constant currents injected into the remaining sections of the laser which results in no power difference between directions for a certain value of reflector current is considered to be a hysteresis-free combination, and is stored. This publication therefore provides a method which enables a laser to be controlled to transmit a certain wavelengths that will not cause the laser to operate in its unstable hysteresis region.
The method disclosed in the above International patent publication detects the regions of instability of a laser solely by means of measuring the optical power or wavelength of the laser. However a problem arises where the power difference is relatively small, as it can be difficult to detect small changes in the power of a laser.
As part of this invention other means of detecting regions of instability are noted, such as side mode suppression of a laser or gain section voltage.
Another problem associated with the above invention is that it requires large changes in current to be made while making measurements. One such significant current change occurs each time the currents injected into the sections of the laser are incremented, so as to provide a new combination of injected different constant currents for which a set of power measurements can be obtained, as during the time these injected currents are being incremented, the reflector current must be adjusted to zero. As the electrical power entering a laser is turned into heat in the passive sections of the laser, it will be appreciated that it is undesirable that this method requires such a large reflector current change, as these large current changes will result in a change of laser temperature.
The above-identified publication additionally requires that all the power measurements are performed while the reflector current is increasing. However, as explained above, an increase in reflector current results in an increase in the temperature of the laser. As the output characteristics of the laser are temperature dependent, it would be advantageous if the laser temperature could be kept constant while performing the measurements.